During a certain time, the so-called recovery time, after the end of the current-carrying interval of a thyristor, the ability of the thyristor to take up off-state voltage is reduced. If it is then subjected to too high an off-state voltage, there is a risk of the thyristor being self-ignited, which may lead to the thyristor being damaged or destroyed. To avoid this risk it is known to use a recovery protection device of the kind referred to above. Such a protection device is known for example, from U.S. Pat. No. 4,084,206. This protection device has one portion positioned at earth potential. To transmit signals between this portion and the control circuits arranged at thyristor potential, there are required two potential separating transmission channels for each thyristor. A typical high-voltage thyristor valve has a large number of series-connected thyristors and therefore the number of necessary transmission channels is large. This has two considerable disadvantages. First of all the great number of transmission channels results in the valve becoming complicated and more expensive. Secondly, it involves a reduction of the reliability of operation of the valve. This reduction is not insignificant since the optoelectronic components (light-emitting diodes) included in a typical transmission channel have a lower reliability and a lower life than the other components included in the control circuits of the valve.
In the known recovery protection device, the protection device is restored by the next ordinary control pulse from the pulse device of the valve. This means that the protection device may be activated only a single time each period, and this fact is a considerable drawback in certain operating cases.
The invention aims to provide a recovery protection device of the kind referred to above, which does not require any potential-separating transmission channels and therefore renders the valve less complicated, resulting in a lower price and a higher reliability in operation. Further, the invention aims to provide a recovery protection device which is ready for operation immediately after having been activated.